My name is Gareth, which tends to be pronounced /ˈgarɛθ/ in Welsh and /ˈgarəθ/ in English. I tend to pronounce it the first way (for no very good reason, apart from habit) when speaking English and other languages, although the phontactics of the language in question occasionally interfere, so I often seem to come out with something like /ˈgaʁɛθ/ (or even /gaˈʁɛθ/) in French. As for spelling: I tend to write it Гарет in Russian and غـاريث in Arabic (at least that’s what I wrote at the top of my Arabic homework). In languages written in the Latin alphabet, however, I always write it Gareth. So I’ve been surprised on two occasions to be told that, in Welsh, it’s written Garedd.
Both times it was an English person who told me this, and both claimed to have been told this by a Welsh person. I have no reason to doubt this. As I understand it, the assertion is that, while most people spell it Gareth, it’s ‘more Welsh’ to spell it Garedd. As far as I know, however, there is no good basis for this assertion.
It’s certainly not the case, as someone I spoke to recently seemed to believe, that the final sound of Gareth is spelt with double d in Welsh. In English, there are two sounds that are represented in writing by th. One is the sound at the start of thigh; the other is the sound at the start of thy. The second sound ([ð]) is written dd in Welsh; the first ([θ]), which is also the sound at the end of my name, is spelt th. So Welsh spelling makes a distinction that English spelling doesn’t, but when the only pair of words where the distinction matters is thy and thigh, there’s not much need to represent it. But if some Welsh people are spelling it Garedd, they’re presumably also pronouncing it /ˈgarɛð/. But why?
Now presumably there’s some etymological claim being made, but the evidence is poor for Garedd being, in some sense, the original form before the English started messing with it. The name first appears (according to all the sources I’ve ever read) in Thomas Malory’s Le Morte d’Arthur. According to the Wikipedia article, the name only became popular in Britain in the 1970s. Malory apparently based the name on Gahariet, which he found in French Arthurian romances. The problem is that it’s not clear where this name came from. The most common suggestion, which probably explains the Garedd thing, is that it comes from some form of the Welsh word now spelt gwaraidd, which means ‘gentle’ or ‘civilised’ (gwareiddiad, a more common word, is Welsh for ‘civilisation’). This is an attractive explanation, bearing in mind the story of Gareth, who was notable for having the soft white hands of someone unused to manual labour, which gave him away when he was disguised as a kitchen boy. But, as far as I can tell, the only evidence for this being the origin is this appropriateness to the story and the assumption that, as it’s an Arthurian name, it must come from Welsh; and this is, granted, the most likely candidate in Welsh. If anyone knows more about it, I’d be glad to hear from them.
But even if that is the real origin of the word, that doesn’t mean that the modern name is ‘correctly’ spelt Garedd (or pronounced /ˈgarɛð/)! Besides, why don’t any of these people go the whole hog and call themselves Gwaraidd or Gwaredd? Just because a name might be derived from a word spelt and pronounced differently from it, that’s no reason to change the name. Most names we use, after all, have changed somewhat since they were first used, and they are often very unlike the common nouns they originated in.
Now people are very welcome to call themselves Garedd if they want to. There’s nothing the slightest bit wrong with that; I just wish they wouldn’t start telling people they’re being more ‘correct’.